1. Field of the Invention
The present invention relates to a device for detecting tire pressure of an automotive vehicle.
2. Description of Related Art
A tire pressure detecting device for an automotive vehicle is a device for watching tire pressure while the vehicle is being driven and for giving a warning to a driver if the tire pressure is decreased abnormally. There have been known two kinds of the tire pressure detecting device. One is a device which detects the tire pressure directly by measuring air pressure in tires, and the other is a device which detects the tire pressure indirectly based on rotational angular velocities of wheels sensed by wheel speed sensors.
An example of the tire pressure detecting device detecting it indirectly is disclosed in JP-A-63-305011. The device disclosed calculates a sum of rotational angular velocities for a pair of wheels located at a diagonal position and compares it with that of the other pair of wheels. If a difference between the two is smaller than a predetermined value, a tire pressure decrease in a particular wheel is judged by comparing an angular velocity of that wheel with an average angular velocity of four wheels. The detecting device of this kind can be made in a simple structure because it does not directly measure the tire pressure but detects it based on the wheel speeds. There is a drawback, however, in this device that detection accuracy of the tire pressure tends to be affected by driving conditions and conditions of a road surface. To reduce the influence from driving conditions and road surface conditions, a tire pressure detecting device disclosed in JP-A-6-286430 prohibits judging the tire pressure under certain conditions. This device calculates a vehicle body speed from an average wheel speed of driven wheels and determines whether the vehicle is under acceleration or deceleration based on a change of the vehicle body speed. If it is determined that the vehicle is accelerating or decelerating, the tire pressure judgment based on the wheel speed is prohibited because it is presumed that there is some skid between the tires and the road surface. Also, a tire pressure detecting device is known in which the tire pressure judgment is prohibited in case a wheel speed difference between driving wheels and driven wheels becomes larger than a threshold value.
However, there is a drawback in the device prohibiting the tire pressure judgment based on the calculated vehicle body speed. That is, there is a situation where the tire pressure judgment has to be prohibited even when the vehicle body speed is constant, if there is a skid between the tires and the road surface. This situation occurs, for example, when the vehicle is driven uphill. A slope sensor is equipped in this device to find out such a situation, which makes the device more complex and expensive. There is also a drawback in the later device prohibiting the tire pressure judgment based on the wheel speed difference between the driving and driven wheels. That is, the tire pressure may not be detected accurately when there is a slight difference in diameters of driving and driven wheels caused by tire abrasion. To cope with this problem, it is known to provide a computer program for learning the tire abrasion during driving. However, this is not practical because performing the learning program requires too much time.